About 70% of more than half a million Implicit Association Tests (IATs) completed by citizens of 34 countries revealed expected stereotypes associating science with male more than with female. We discovered that nation-level implicit stereotype predicted nation-level sex differences in 8th-grade science and mathematics achievement. Self-reported stereotypes did not provide additional predictive validity of the achievement gap. We suggest that implicit stereotypes and sex differences in science participation and performance are mutually reinforcing, contributing to the persistent gender gap in science engagement.